1. Field
The invention is concerned with a method of creating MEMS structures by selectively etching a silicon wafer that is patterned by using a masking layer for defining the structural features of a MEMS device. The invention is also concerned with the use of the method.
2. Description of the Related Art
A micro-electromechanical system (MEMS) device has moving (inertial) elements under control of integrated microelectronics and contains micro-circuitry on a tiny silicon chip into which some mechanical device, such as a microsensor, and/or a microactuator has been manufactured. These microsensors and microactuators constitute the functional elements of MEMS devices. The physical dimensions of MEMS devices can vary from below one micron to several millimeters.
MEMS devices convert a measured mechanical signal into an electrical signal. MEMS sensors measure the mechanical phenomenon and the electronics then process the information derived from the MEMS sensors and through some decision making capability direct the actuators to respond by e.g. moving, positioning, or regulating in order to thereby control the environment for some desired outcome or purpose. MEMS devices can thus comprise both drive elements and sensing elements to perform specific functions.
Examples of systems fabricated using MEMS technology are pressure sensors, accelerometers for measuring acceleration of moving objects and gyroscopes for measuring angular velocity of rotating objects.
MEMS devices may be capacitive or they may make use of piezoelectric transduction.
A key element in a capacitive MEMS device is a variable capacitor formed between a stationary electrode and a movable electrode attached to a flexibly suspended proof mass. The movable electrode deflects in response to acceleration in an accelerometer or Coriolis force exerted on the proof mass when an angular velocity is applied to a gyroscope and used for measuring this angular velocity. The amount of deflection can be sensed from changes in capacitance caused by the changes in the gap between the two electrodes due to deflection.
Accelerometers are acceleration sensors. An inertial mass suspended by springs is acted upon by acceleration forces that cause the mass to be deflected from its initial position. This deflection is converted to an electrical signal, which appears at the sensor output. The application of MEMS technology to accelerometers is a relatively new development.
An accelerometer comprises a proof mass, one side of which is fixed to a carrier while the other is flexibly suspended by a membrane or a cantilever beam, for example. The accelerometer further comprises means for detecting the movement of the proof mass caused by the effect of acceleration. This constitutes a sensor, which senses acceleration force.
Inertial sensors are a type of accelerometer and are one of the principal commercial products that utilize surface micromachining.
When things rotate around an axis they have angular velocity. Gyroscopes, or gyros, are devices that measure or maintain rotational motion. In MEMS devices, vibration is typically used as primary motion of the gyroscope rather than rotation. In a vibrating sensor of angular velocity, i.e. a gyroscope, a certain known primary or seed motion is induced and maintained in the sensor. The desired motion to be measured by means of the sensor is then detected as a deviation of the primary motion.
When MEMS technology is implemented as gyroscopes, these have a structure suspended above a substrate and associated electronics that causes the primary motion, senses movement of the suspended structure and deliver the sensed movement to an external computer. The computer processes the sensed data to calculate the property being measured. In some embodiments, the substrate may be formed by a silicon wafer called the handle wafer on which the device wafer is attached to.
Structures for vibrating gyroscopes are formed, for example, by etching a semiconductor wafer to form a proof mass used as a reference in the measurement. The proof mass is suspended by a spring system, such as elastic beams, to a substrate that may be considered as a fixed structure in relation to the proof mass. An electronic drive circuit which may be on the same substrate applies an alternating drive current to driving electrodes which vibrate the proof mass in a drive direction. The electrical drive mechanism vibrates the proof mass along a drive axis and the electrodes build a capacitance together with the proof mass for detecting movement of the proof mass along a sense axis perpendicular to the drive axis. A triple axis MEMS gyroscope, can measure rotation around three x, y, and z axes, while single and dual axis gyros measure the rotation around one or two of these axis correspondingly.
The electrodes detect displacements of the proof mass in a sensing direction orthogonal to the drive direction. The vibrating gyroscope thus rely on the vibration of this proof mass in one direction as the primary motion and in detecting the movement caused by Coriolis force, generated in the perpendicular direction by the rotational speed. Conversion of rotation to Coriolis force is the basis of the operation of a gyroscope.
The production process and the technology used for producing the structures with the springs and the beams in MEMS based gyroscopes often lead to quadrature errors, such as errors caused by driving the vibrating proof mass along a direction which is not exactly perpendicular to the direction along which the Coriolis movement is measured. The component of the vibrating drive movement in the sense direction generates an output signal caused by the Coriolis force.
One of the most significant problems in micromechanical vibrating sensors of angular velocity is the so called quadrature signal, which is caused by poor dimensional precision in the structures. The quadrature output signal is in phase with the drive signal used for driving the proof mass, while the component for this output signal due to the Coriolis force is shifted by 90 degrees.
In the sensor, the quadrature signal can be compensated for by using electric forces, such as e.g. feedback compensation, feed-forward compensation, or other electrical compensation.
Compensation by means of electric forces, however, constitutes a challenge to the sensor's electronics requiring either accurate phase control or, possibly, large voltages and separate structures within the sensor.
These miniaturized sensors, actuators, and structures can all be merged onto a common silicon substrate or on separate silicon substrates along with integrated circuits (microelectronics). While the electronics are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using micromachining processes that selectively etch away parts of a silicon semiconductor wafer or add new structural layers to form the mechanical and electromechanical devices. The wafer is patterned and etched to define the structural features of the sensor in the semiconductor layer. The wafer serves as the substrate for the microelectronic devices built in and over the wafer and, in addition to patterning and etching, undergoes many other microfabrication process steps, such as doping or ion implantation, and deposition of various materials. Finally the individual microcircuits are separated into dies and packaged.
Die refers to one small block of the semiconducting material, on which a given functional circuit, a chip, is fabricated. In the manufacturing of the micro-electronic devices, each individual die contains one of the integrated circuits. During manufacturing, a wafer with up to thousands of circuits is cut into rectangular pieces, each called a die. The integrated circuits are produced in large batches on a single wafer.
Device wafer refers to the semiconductor wafer that is used for manufacturing the functional, inertial parts of a MEMS device. Device wafer may further include at least some electrical parts of the MEMS device.
Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete. For many etch steps, part of the wafer is protected from the etchant by a “masking” material which resists etching. The masking material is e.g. a photoresist which has been patterned using photolithography. The patterning shows which parts of the wafer should be etched.
In anisotropic etching, the etching rate is different in horizontal and vertical direction. Bias refers to the difference in lateral dimensions between the feature on mask and the actually etched pattern caused by undercut, which refers to a portion that is etched away under the mask.
The profile of the etched structures has a big impact on the performance of the MEMS device. A typical non-ideality in especially Deep Reactive Ion Etched (DRIE) structures causing problems is that some etches undercut the masking layer and form trenches with sloping sidewalls. The distance of undercutting is called bias. The undercut problem is even more difficult to solve if its extent varies within the structure or within the area of the semiconductor wafer.
Undercut can be defined as the difference between the mask intended to define the etching boundaries and the actual etched dimensions.
Usually, the undercut is compensated by using mask bias, which means making the mask dimensions larger than the intended trench dimension to compensate for the undercut. However, the biasing of the mask does not provide a complete solution because the DRIE undercut varies across the wafer. This is because there is a radial distribution coming from the geometry of the DRIE tool. Furthermore, some MEMS devices, like z-axis gyroscopes, are more critical to dimension accuracy within one die than to die-to die variations. This is because beam variation within one die causes the primary motion to differ from the designed direction resulting in the so called quadrature signal. The quadrature signal of a z-axis gyroscope is highest in such a wafer area where the dimension change rate, as a function of position, is highest.
An attempt to provide a structure of a vibrating sensor of angular velocity, in which the compensation for the quadrature signal is implemented directly by mechanical design, without electric forces is disclosed in U.S. Pat. No. 8,210,039.
U.S. Pat. No. 8,043,973 discloses a method for mask overhang reduction by a process design comprising the use of two masking layers to reduce lateral substrate undercut.
U.S. Pat. No. 7,214,559 discloses a method for fabricating a vertical offset structure by using several etching steps.